Adding support for optional nick prefix to `get_replies`

See https://github.com/cinchrb/cinch/blob/master/lib/cinch/message.rb#L158

The `Message.reply` function takes an optional second argument (prefix = false) to automatically prepend the nick of the user who issued the command. Since it's missing in the get_replies redefinition it was making it tough to test my plugins that make use of that feature.